Ladder filter with piezoelectric resonators each having a plurality of layers with internal electrodes

Abstract

A ladder type filter achieves a substantial reduction in distortion in filtering characteristics. The ladder type filter includes a supporting substrate formed of an insulating material. Supporting members are attached to lands which are formed by portions of pattern electrodes disposed on the supporting substrate. A plurality of piezoelectric resonators are mounted on the supporting members and connected in a ladder arrangement to define a ladder type filter. Each piezoelectric resonator has a base member in which piezoelectric layers and internal electrodes are laminated. External electrodes formed on one side surface of the base member are connected to the internal electrodes disposed in the base member. The width w and the thickness t of the piezoelectric resonators and the number n of laminated layers each comprising one of the piezoelectric layers and one of the internal electrodes disposed adjacent to the corresponding piezoelectric layer are selected so as to satisfy 1≦log 10 (n 2 wt)≦3.

What is claimed is: 
     
       1. A ladder type filter comprising: a plurality of piezoelectric resonators connected in series and in parallel in a ladder arrangement, each of said piezoelectric resonators including a base member having a plurality of piezoelectric layers and a plurality of the internal electrodes alternately laminated along a longitudinal direction of the base member, said piezoelectric resonator being arranged to vibrate in a longitudinal direction of said base member by application of an electric field between each adjacent pair of said electrodes; wherein   if a width of each of said piezoelectric resonators is w, a thickness of each of said piezoelectric resonators is t, and the number of laminated layers each including one of said piezoelectric layers and one of said internal electrodes disposed adjacent to a corresponding one of said piezoelectric layers is n, then an inequality 1≦log 10  (n 2  wt)≦3 is satisfied.   
     
     
       2. A ladder type filter according to claim 1, wherein each of said piezoelectric layers is polarized along the longitudinal direction of said base member. 
     
     
       3. A ladder type filter according to claim 1, wherein adjacent pairs of said piezoelectric layers located on opposite sides of each of said internal electrodes are polarized in opposite directions. 
     
     
       4. A ladder type filter according to claim 1, wherein each adjacent pair of said internal electrodes are respectively connected to two external electrodes which are disposed on one side surface of said base member. 
     
     
       5. A ladder type filter according to claim 1, wherein a plurality of said piezoelectric resonators are connected in series and in parallel in a ladder arrangement on an insulating substrate having pattern electrodes disposed on a surface of said insulating substrate, and a cap is mounted on said insulating substrate so as to cover said piezoelectric resonators.   
     
     
       6. A ladder type filter according to claim 1, further comprising a substrate and a plurality of support members for supporting said plurality of piezoelectric resonators. 
     
     
       7. A ladder type filter according to claim 6, wherein said substrate has a plurality of pattern electrodes disposed on a surface thereof and said plurality of support members are mounted on said plurality of pattern electrodes to electrically connect said plurality of piezoelectric resonators to said plurality of pattern electrodes. 
     
     
       8. A ladder type filter according to claim 6, wherein said plurality of support members each include an electroconductive adhesive material. 
     
     
       9. A ladder type filter according to claim 6, wherein each of said plurality of piezoelectric resonators has a node point defined at a center thereof and each of said plurality of piezoelectric resonators is supported on said substrate at said center thereof. 
     
     
       10. A ladder type filter according to claim 1, wherein said piezoelectric base member includes an active portion and at least one non-active portion. 
     
     
       11. A ladder type filter comprising: a plurality of piezoelectric resonators connected in series and in parallel in a ladder arrangement, each of said piezoelectric resonators including a base member having an active portion and at least one non-active portion and including an integral unit containing a plurality of piezoelectric layers and a plurality of internal electrodes, said piezoelectric resonator being arranged to vibrate in a longitudinal direction of said base member; wherein   if a width of each of said piezoelectric resonators is w, a thickness of each of said piezoelectric resonators is t, and the number of laminated layers each including one of said piezoelectric layers and one of said internal electrodes disposed adjacent to a corresponding one of said piezoelectric layers is n, then an inequality 1≦log 10  (n 2  wt)≦3 is satisfied.   
     
     
       12. A ladder type filter according to claim 11, wherein each of said piezoelectric layers is polarized along the longitudinal direction of said base member. 
     
     
       13. A ladder type filter according to claim 11, wherein adjacent pairs of said piezoelectric layers located on opposite sides of each of said internal electrodes are polarized in opposite directions. 
     
     
       14. A ladder type filter according to claim 11, wherein each adjacent pair of said internal electrodes are respectively connected to two external electrodes which are disposed on one side surface of said base member. 
     
     
       15. A ladder type filter according to claim 11, wherein a plurality of said piezoelectric resonators are connected in series and in parallel in a ladder arrangement on an insulating substrate having pattern electrodes disposed on a surface of said insulating substrate, and a cap is mounted on said insulating substrate so as to cover said piezoelectric resonators.   
     
     
       16. A ladder type filter according to claim 11, further comprising a substrate and a plurality of support members for supporting said plurality of piezoelectric resonators. 
     
     
       17. A ladder type filter according to claim 16, wherein said substrate has a plurality of pattern electrodes disposed on a surface thereof and said plurality of support members are mounted on said plurality of pattern electrodes to electrically connect said plurality of piezoelectric resonators to said plurality of pattern electrodes. 
     
     
       18. A ladder type filter according to claim 16, wherein said plurality of support members each include an electroconductive adhesive material. 
     
     
       19. A ladder type filter according to claim 16, wherein each of said plurality of piezoelectric resonators has a node point defined at a center thereof and each of said plurality of piezoelectric resonators is supported on said substrate at said center thereof. 
     
     
       20. A ladder type filter according to claim 11, wherein said plurality of piezoelectric layers and plurality of the internal electrodes of the piezoelectric base member are alternately laminated along a longitudinal direction of the base member to form said integral unit.